It is necessary in automatic transmissions that a transmission output shaft be blocked when engaging the “P” position of the selector lever in order to prevent the vehicle from rolling away. In the novel, so-called shift-by-wire shifting devices, the selector lever position at the shifting device is detected and electrically transmitted to the transmission control device, and there is no longer any mechanical connection between the shifting device and the transmission. The transmission control device implements the driver's desire by energizing an actuator (for example, an electric motor on the selector shaft) or directly electromagnetic valves in the transmission.
Such a shifting device with parking brake, which is engaged by means of a pretensioned spring and can be disengaged by admitting the system pressure of the hydraulic transmission control, is described in the German Offenlegungsschrift No. DE 196 43 812 A1.
The drawback of such solutions with spring-loaded mechanism is that auxiliary energy is always necessary to disengage the parking brake. If this is not available, for example, due to a defect in the hydraulic system of the transmission, due to the engine not running or due to the battery having been removed, an emergency unlocking is to be provided in order to make it possible to push the vehicle, for example, in an emergency or in the repair shop. Making such an emergency unlocking is correspondingly complicated and expensive.
Another drawback of such a solution with spring-loaded mechanism is that the engagement of the parking brake cannot be triggered under all circumstances by means of the selector lever. If the electric communication between the shifting device and the transmission is interrupted, the parking brake can be engaged only by interrupting the supply of the auxiliary energy. This requires, for example, that the engine must be turned off in order to allow the hydraulic system pressure to collapse, as a result of which the spring-loaded mechanism will engage the parking brake.
A possibility of actively engaging and disengaging the parking brake under all circumstances and at any time can be achieved by the use of a cable, which is arranged between the shifting device and the transmission, as in the convectional, mechanically connected automatic shifting devices. This cable is then moved by the selector lever during each shifting movement, but it is used only to engage and disengage the parking brake. The positions P, R, N and D are, however, transmitted electrically now. Since this cable is directly or indirectly connected to the selector lever, it is also moved along during the movement between all positions in which no parking brake is activated or deactivated, i.e., it travels a relatively great distance, whereas only a relatively short path would be necessary for engaging and disengaging the parking brake. The friction generated during the unnecessary movement of the cable has an adverse effect on the feeling of shifting during shifting in the non-P positions.
Another possibility, which solves the problem of the cable being moved continuously, is described in DE 101 25 526 A1. The coupling and uncoupling of the cable for the parking brake and the selector lever is achieved here by a catching device, which can be actuated manually indirectly or directly. If the cable to the parking brake is connected to the selector lever, this connection must again be severed by a manual action in order for the selector lever to be able to move freely.